1. Field of the Invention
The present invention relates to a process for producing silicon carbide fibers. More particularly, the present invention relates to a process for producing silicon carbide fibers directly from a carbon-supply source consisting of activated carbon fibers and a silicon-supply source consisting of a powder of at least one member selected from elemental silicon and silicon oxides.
2. Description of the Related Art
A simple process for producing silicon carbide fibers having a high mechanical strength by reacting porous carbon fibers with a gaseous silicon monoxide (SiO) at a temperature of 800 to 2000.degree. C. is disclosed in Japanese Unexamined Patent Publication No. 6-192,917.
Also, Japanese Unexamined Patent Publication No. 7-97,281 discloses a process for producing silicon carbide material in the form of a sheet or a three-dimensionally constructed article, by reacting a mass of porous carbon fibers in the form of a sheet or a three-dimensionally constructed article, for example, an article having a honeycomb structure, with gaseous silicon monoxide (SiO) at a temperature of 800 to 2000.degree. C.
By the above-mentioned processes, the silicon carbide fibers or a shaped article consisting thereof, having a high heat resistance and an excellent mechanical strength can be easily produced. However, these processes are disadvantageous in that when a high efficiency of the siliconizing reaction is required, the reaction must be carried out under a reduced pressure and thus the reaction equipment must be complicated, and the reaction for producing the silicon carbide fibers is difficult to carry out at high efficiency.
Japanese Unexamined Patent Publication No. 1-111,800 discloses a process for producing silicon carbide whiskers. In this process, a first reaction region is formed from carbon or a carbon precursor and fine particulate silicon dioxide particles uniformly mixed with each other at a mixing weight ratio of about 5:1 or more, a second reaction region is formed closely adjacent to the first reaction region from porous and fibrous non-fusible carbon precursor or activated carbon having a porosity of 70% by volume or more and a length of 200 .mu.m or more, the first and second reaction regions are heated to a desired reaction temperature of 1200 to 1700.degree. C. in a non-oxidative atmosphere, thereby to produce silicon monoxide in the first reaction region, to immediately cause the silicon monoxide to diffuse into the second reaction region and react with the above-mentioned activated carbon or carbon contained in the carbon precursor in the second reaction region to produce silicon carbide. In this process, during the production of silicon carbide, separately, a whisker-production catalyst which is continuously applicable, is supplied in and around the second reaction region in which at least the silicon monoxide and carbon react with each other.
This process can produce silicon carbide whiskers. In this process a silicon monoxide gas is produced from a mixture of extremely fine silicon dioxide particles and carbon in a reaction region, introduced into another reactive region filled with activated carbon in a porous filling condition and containing a whisker producing catalyst and passed through the other reactive region so as to react with the activated carbon or the carbon derived from the carbon precursor in the presence of the catalyst, for example, an iron compound, and to produce the silicon carbide whiskers on the carbon surface.
The whiskers are a crystalline material consisting of silicon carbide (SiC) and are in the form of fine fibers having a diameter of 0.5 to 1 .mu.m and an aspect ratio of 20 to 100. Therefore, the whiskers have a very high tensile strength, modulus of elasticity and ultimate elongation. However, as a practical reinforcing material, the silicon carbide whiskers are not always useful in comparison with the silicon carbide fibers produced by reacting the activated carbon fibers with a silicon monoxide gas. The term "aspect ratio" used herein is a ratio of a length to a diameter of a fiber.
Namely, when carbon is heated in the presence of a silicon carbide gas, and resultant carbon compounds generated from the carbon, such as carbon monoxide (CO), carbon dioxide (CO.sub.2) and methane (CH.sub.4), react with the silicon monoxide, the resultant silicon carbide is deposited on the carbon surfaces and grows in the form of whiskers from the carbon surfaces. Therefore, the silicon carbide whiskers are quite different from the above-mentioned silicon carbide fibers directly converted from the activated carbon fibers per se by a reaction of the activated carbon fibers with silicon monoxide.
In the mentioned process for producing silicon carbide fibers by the reaction of the activated carbon fibers with a silicon monoxide, the silicon supply source material and the activated porous carbon fibers are placed separately from each other in a reaction furnace, and heated to generate the silicon monoxide gas, and the silicon monoxide gas is brought into contact and reacted with the activated carbon fibers, to convert the activated carbon fibers directly to silicon carbide fibers. This conventional process is disadvantageous in that
(1) since the reaction of the porous activated carbon fibers with the silicon monoxide gas progresses with penetration of the silicon monoxide gas into the inside of the porous activated carbon fibers, the efficiency of the reaction can be enhanced only by making the specific surface area of the porous activated carbon fibers as large as possible;
(2) even if the specific surface area of the porous activated carbon fibers is sufficiently large, when the reaction is carried out at a high temperature of 1000.degree. C. or more, the fine pores distributed in the activated carbon fibers are crushed so as to decrease the specific surface area of the porous fibers, and thus the reaction efficiency is decreased; PA1 (3) if the reaction atmosphere is not established under a reduced pressure, a concentration of the silicon monoxide necessary to effect the reaction is difficult to obtain, and thus the reaction efficiency becomes very low; and PA1 (4) due to the low reaction efficiency, when the porous activated carbon fibers to be subjected to the reaction with the silicon monoxide gas are in the form of a bulky sheet or other three-dimensional shaped article, the silicon carbide-producing reaction cannot be uniformly carried out, and thus the resultant silicon carbide fibers contain non-reacted carbon portions. PA1 mixing a silicon-supply source powder comprising at least one member selected from silicon and silicon oxides with activated carbon fibers having a fiber thickness of 1 to 20 .mu.m and a specific surface area of 300 to 2000 m.sup.2 /g determined by the BET nitrogen absorption method; and PA1 heating the resultant mixture at a temperature of 1200 to 1500.degree. C. in an atmosphere substantially free from substances reactive with carbon, silicon, silicon oxides and silicon carbide at the above-mentioned temperature to directly convert the activated carbon fibers to silicon carbide fibers.
Accordingly, it has been strongly demanded to provide a new process for producing silicon carbide fibers or shaped articles thereof having high mechanical strength and heat resistance from activated carbon fibers or shaped articles thereof and a silicon-supply source material under the ambient atmospheric pressure and at a relatively low temperature with a high reaction efficiency.